1. Field of the Invention
This invention relates generally to the field of optical telecommunications, and in particular, to optically demodulating differential-phase-shift-keyed (DPSK) signals in a wavelength-division-multiplexed (WDM) communication environment.
2. Description of the Related Art
In state-of-the-art, wavelength-division-multiplexed optical communication systems, multiple, individually modulated wavelengths of light are multiplexed into a single fiber and routed to a remote receiving location. At the receiving location, it is common to separate the WDM channels using an optical wavelength-demultiplexing filter in order to route each channel to its own, dedicated demodulator.
In a system that employs differential-phase-shift-keyed modulation, a dedicated DPSK demodulator is used within the receivers of each WDM channel. A DPSK demodulator may be implemented using an optical delay interferometer (DI) (e.g., a Mach-Zehnder interferometer) followed by an optical-to-electrical (O-E) converter. The function of the DI is to convert a phase-coded optical signal into an intensity-modulated signal before the signal is presented to the O-E converter. The function of the O-E is to convert the resulting optical intensity modulation to an electrical current or voltage that may be further processed in the electrical domain.
The DI operates by splitting the DPSK input signal into two components and delaying one component with respect to the other by a time delay Td, where the time delay is usually set to be an integer multiple of the bit duration T. In most cases, but not exclusively, the delay Td is chosen to equal the bit duration T of the modulated bit stream. The two relatively delayed components of the phase-coded input signal are then combined at an output coupler of the DI. The relative optical phase difference between the two signals within the DI is set to produce, in the absence of phase changes, constructive interference at one of the DI output ports and destructive interference at the other. When single-ended detection is employed by the demodulator, only one output is brought out of the DI to an O-E converter that is typically implemented using a photodiode and associated circuitry. When balanced detection is employed by the demodulator, both outputs are brought out of the DI to two such O-E converters.
In a single-ended implementation, a single output port of the DI is coupled to a single photodiode detector and circuitry to translate the optical intensity out of the DI into a recovered bit stream.
In a balanced implementation, each of two ports out of the DI is coupled to its own photodiode, and the difference between the electrical outputs of the photodiode circuits are used to recover the bit stream.
For a DPSK-modulated WDM system of N channels, it is typical to employ an N-channel optical wavelength-demultiplexing filter, followed by N dedicated DPSK demodulators, one for each receiver.